1. Technical Field
The present invention relates to a negatively chargeable spherical toner used in electrophotography, and a color image forming apparatus, and a process for producing a negatively chargeable spherical toner.
2. Related Art
In the electrophotography, after an electrostatic latent image formed on a latent image carrier provided with a photoconductive material is developed by using a toner containing a colorant, transferred to an intermediate transfer medium, further transferred to a recording material such as paper and then fixed by heat, pressure or the like, to form a copied material of a printed material. In such case as utilizing the latent image carrier, for example, in Reference 1, it is described that, in the toner mother particles, using an alumina fine particle as an external additive particle, holding the surface of a latent image carrier clean all the time by an abrasive action against the surface of a latent image carrier and by such procedure, such problems as fogging and scattering of the toner are tried to be prevented and a stable image can be formed and further, an alumina fine particle having a large diameter of from 0.1 to 1.0 μm is preferably defined as the alumina fine particle. However, since the alumina fine particle having a large diameter tends to have a smaller adhesive force to the toner mother particle than that of the alumina having a small diameter from a relation of a mass thereof, there is a problem that it is liable to be dropped off. Particularly, the alumina fine particle is high in hardness and when the amount of the alumina fine particle freed from the toner mother particle is large, there is a problem that various members of the apparatus are abraded. Further, when such amount of the alumina fine particle freed is large, it gives an influence on the powder characteristics or the electrostatic properties and particularly, when a continuous printing is performed, a behavior of the toner is changed in a discontinuous manner, and accordingly there is a problem that an image quality such as image density or color reproducibility to be obtained becomes uneven, particularly, in a full-color image. Further, when the toner in which the alumina fine particle having a large particle size becomes an external additive is tried to be applied to a non-contact developing method, in the case in which the amount of the alumina fine particle having a large particle size freed from the toner mother particle is large, it has been found that various types of problems are generated such that a scattering property is deteriorated and stability of the printed image is reduced and the like.
Further, as for a conventional technique regarding the external additive, externally adding three types of external additives having different particle sizes from one another to the toner mother particle is described in, for example, Reference 2 and JP 11-184144 A; externally adding an external additive having a large particle size and a charged polarity reverse to that of the toner mother particle or an abrasive is described in Reference 3 or JP 2003-322998 A; and externally adding an inorganic fine particle having a large particle size is described in Reference 4. In any of such cases as described above, when a long-term continuous printing is performed, the external additive is freed from the surface of the toner and then the freed external additive adheres to the surface of the image carrier or the surface of the intermediate transfer medium to cause a problem of the increase in the fogging or reversal transferred toner or a problem of resulting in the decrease in transfer efficiency. This phenomenon is that the freed external additive having a reverse polarity or a negatively chargeable toner left from being transferred firmly adheres on the latent image carrier and is not transferred to the intermediate transfer medium and further, there is a problem that the freed external additive promotes abrasion of the surface of the developing member. For example, as far as a non-magnetic monocomponent developing roller is concerned, convex and concave (Rz) thereof becomes small, to cause a change of the amount of the toner to be transported.
Then, as for the documents for restricting the amount of the external additive to be freed, those as described in, for example, Reference 5, and JP 2002-189309 A, JP 2002-207314 A, JP 2002-236386 A, JP 2002-258522 A, JP 2003-207942 A, JP 2003-280240 A, JP 2003-280253 A and JP 2004-184719 A are known. Any one of them enhances a cleaning performance, prevents abrasion of the latent image carrier, improves an image quality or enhances flowability or prevents filming or abrasion, but does not positively prevent the external additive having a large particle size from being freed from the surface of the toner mother particle and as a result, there still exist disadvantages in, for example, stabilizing property of the printed image, prevention of various types of filming and abrasion, and enhancement of charge stability of the toner.
Further, the definition of the work function of the toner or the external additive and trying for improving the image quality or stabilizing the charge properties are described in, for example, Reference 6, and JP 11-174726 A and JP 2003-202696 A, and still further, an attempt to realize a stabilized charge properties or to enhance the transfer efficiency by defining the work function of the external additive is described in Reference 7. In any of such cases as described above, after a continuous printing of several ten thousands of pages is performed, the initial toner properties can not be maintained and further, it is insufficient for providing a stabilized color image by preventing filming and abrasion.
Further, as shown in FIG. 9, the Henschel mixer has a mixing processing tank 101 in the cylinder shape and a stirring blade rotating with a high speed on the bottom of the mixing processing tank, and promotes the mixing by repetition of the upward and downward motion such that the material to be treated is transferred with a centrifugal force generated by the lower blade 105 rotating with a high speed on the bottom of the mixing processing tank to the wall of the processing tank, and the material to be treated is slipped down on the inclined surface formed by the deposition of the material to be treated itself by gravity when the influence of the upward force by the centrifugal force, and then is again upwardly moved with the centrifugal force generated by the blade rotating with a high speed. Further, it may be conceived that the stirring and dispersion may be promoted by rotating the upper blade 110 while the material to be treated is slipped down on the inclined surface by the deposition of the material to be treated itself. However, in such the Henschel mixer, the inclined surface formed by the deposition of the material to be treated itself is slipped won as it is by gravity, and thus rotation rarely occurs. Thus, this easily causes the same parts between the particles to be brought into contact, thus there being a problem that a desired dispersed adherence, that is, a homogeneous adherence is hardly achieved.
Meanwhile, it is known that a mixing processing tank in the spherical shape is used instead of the above Henschel mixer (References 8 and 9), which has a problem that when a toner mother particle having a high degree of circularity is employed in order to increase the transfer efficiency, the rotational property is excellent, however the surface area thereof is relatively small as compared with the irregular shaped toner, as well as the convex and concave of the surface is low, thus resulting in the increase in the amount of the external additive to be freed, which is problematic.                References as cited herein are as follows:        Reference 1: JP 8-69123 A        Reference 2: JP 63-289559 A        Reference 3: JP 2002-318467 A        Reference 4: JP 2003-322998 A        Reference 5: JP 2001-117267 A        Reference 6: JP 6-332236 A        Reference 7: JP 2003-295503 A        Reference 8: JP 8-173783 A        Reference 9: JP 2002-268277 A        